Circuit arrangement for a brake system with electronic brake force distribution control

ABSTRACT

A circuit arrangement for a brake system with electronic brake force distribution control includes circuits which determine the deceleration of the rear wheels and the deceleration of a vehicle or a corresponding reference quantity. The discrepancy of the filtered rear-wheel deceleration from the vehicle deceleration is evaluated for brake force distribution control. To this end, an output signal is produced by way of a differentiator and integrator comparing the rear-wheel deceleration and the vehicle deceleration, and the output signal is evaluated for the control of the braking pressure in the rear-wheel brakes.

This application is the U.S. national-phase application of PCTInternational Application No. PCT/EP95/01531.

BACKGROUND OF THE INVENTION

The present invention relates to a circuit arrangement for a brakesystem with electronic control of the brake force distribution to thefront axle and the rear axle, hereinbelow referred to as EBV, includingelectrically controllable hydraulic valves inserted in the pressurefluid conduits leading to the rear-wheel brakes and in return conduits,wheel sensors to determine the wheel rotational behavior, and anelectronic circuit which evaluates the sensor signals, generates signalsto actuate the hydraulic valves and includes circuits which determinethe deceleration of the rear wheels and the deceleration of the vehicleor a reference quantity representative of the deceleration of thevehicle in approximation, and which compare these deceleration values.

German patent application No. 33 06 611 (GB-A-2 135 413) discloses acircuit arrangement of this type. This application relates to a methodof controlling the brake force distribution which is based on measuringand comparing the brake slip on the front wheels and the rear wheels.The brake slip on the rear wheels is limited to the value of the brakeslip on the front wheels or to a somewhat smaller value. The brakingpressure on the rear axle is reduced in response to a signal which isindicative of an imminent locked condition of the rear wheels andproduced by comparison of the rear-wheel deceleration with the vehicledeceleration.

German patent No. 33 23 402 discloses an anti-lock brake systemincluding electromagnetically operable hydraulic valves which are alsoused to control the brake force distribution. For this purpose, an inletvalve is inserted into each pressure fluid conduit to the rear-wheelbrakes. Actuation of the inlet valve permits controlling the pressureincrease in the rear-wheel brakes also in a so-called partial brakingoperation, i.e., prior to the commencement of anti-lock control. Tocontrol or adjust the brake force distribution, an outlet valve can beused to reduce the braking pressure in the rear-wheel brakes. It ispreferred to limit the brake slip on the rear wheels to a definedpercentage, preferably 85-97% of the brake slip of the front wheels, byactuation of the rear-wheel valves.

A brake system with electronic brake force distribution (EBV) of thistype obviates the need for mounting a conventional mechanical orhydraulic braking pressure reducing valve or braking pressure controlvalve which has previously been used to prevent overbraking of the rearwheels due to a static or dynamic axle load shift. This reduces theexpenditure in manufacturing the brake system. In addition, thecomponents which are required in an electronic anti-lock system, inparticular the wheel sensors and the hydraulic valves, may also be usedfor the EBV function.

A shortcoming of these known brake systems with EBV function is that thewheel speed must be measured with great accuracy to determine the actualslip. A defined amount of slip already is involved due to the inevitabledifferences in the rolling circumferences of the individual wheels.Also, differences in wheel speeds occur during cornering, which thecontrol erroneously interprets as slip. Upon failure of one of thefront-wheel brakes, special provisions are required when an EBV functionon the basis of the difference between the slip of the front wheel andthe rear wheel is performed to prevent the braking pressure in therear-wheel brakes from being restricted to an insufficient amount ofbraking pressure.

SUMMARY OF THE INVENTION

An object of the present invention is to overcome the disadvantages ofknown brake systems with the EBV function as described hereinabove. Ithas been found that this object can be achieved by a circuit arrangementof the previously mentioned type. According to the specialcharacteristics of the circuit arrangement, the discrepancy of therear-wheel deceleration from the vehicle deceleration or from thereference quantity representative of the deceleration of the vehicle inapproximation is evaluated for the control of brake force distribution.

Thus, the present invention includes an electronic control of the brakeforce distribution which is virtually independent of the instantaneouswheel slip and, more particularly, of the slip difference frontaxle/rear axle. The previously necessary high demands on measuring andcalculating the wheel speeds are thereby reduced. Upon failure of afront-wheel brake, the measured quantity for the vehicle deceleration orthe corresponding reference quantity remains correct, at least inapproximation, whereby the EBV function is preserved.

In a preferred aspect of the present invention, the wheel sensor signalrepresentative of the speed of a rear wheel is sent to a low-pass filterby way of a differentiating circuit. The output signal of the low-passfilter is compared with the vehicle deceleration or the referencequantity by a differentiator and integrator. The output signal of thedifferentiator and integrator is produced according to the relation

    x(t)=∫(b'.sub.R -b.sub.Fz)dt.

In this relation,

b'_(R) is the filtered deceleration signal of the right (b'₃) or left(b'₄) rear wheel,

b_(FZ) is the vehicle deceleration or a reference quantity inapproximation of the vehicle deceleration.

The braking pressure in the associated rear-wheel brake is reduced, keptconstant or reincreased as a function of the output signal of thedifferentiator and integrator. The magnitude of the output signal isdecisive which is compared with predetermined limit values.

The present invention will be explained in detail hereinbelow, makingreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a schematically simplified view of the most importantcomponents of an embodiment of a brake system with anti-lock control(ABS) and electronic brake force distribution control (EBV).

FIG. 2 is a schematically simplified view of an electronic circuitarrangement of the present invention to control the brake forcedistribution for the brake system in FIG. 1.

FIGS. 3A, 3B, 3C are waveform diagrams useful to explain the operationof the circuit arrangement of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 serves to explain the principal structure of a brake system withABS and EBV functions. The wheel brakes of a vehicle in the embodimentshown are connected diagonally to the two hydraulic circuits I, II of apedal-operated braking pressure generator 1. The front wheels aredenoted VL, VRand the rear wheels are denoted HR, HL in FIG. 1.Electrically operable inlet valves 3-6, open in the inactive position,are inserted in each of the pressure fluid conduits leading from themaster cylinder 2 of the braking pressure generator 1 to each individualwheel brake. Outlet valves 8-11, which are also operable electricallyand closed in the inactive position, are interposed in return conduitswhich lead from the wheel brakes of the individual wheels to a hydraulicpump system 7. Of course, electronic brake force distribution control ispossible also in brake systems having a different allotment of thehydraulic brake circuits.

Only the hydraulic valves 4, 9; 6, 11 associated with the rear-wheelbrakes are required for the electronic control of brake forcedistribution. All inlet valves 3-6 and outlet valves 8-11, respectively,are required for anti-lock control.

Each vehicle wheel is equipped with a wheel sensor S1 to S4. The outputsignals of the wheel sensors are evaluated in a control unit 12 whichincludes the electronic circuits for the brake force distributioncontrol and anti-lock control. Also, the signals to control thehydraulic valves 3-6 and 8-11, connected to the outputs A of the controlunit 12, are produced in the control unit 12. Reference character "E"designates sensor inputs to the control unit 12.

The conditioned sensor signals S1 to S4, as input signals S1' to S4',are sent to the circuit of FIG. 2, which is part of the control unit 12of FIG. 1. The signals S1' and S2' indicate the rotational behavior ofthe front wheels and the signals S3' and S4' indicate the rotationalbehavior of the rear wheels. A vehicle reference speed, which representsthe vehicle speed in approximation, is produced by logically linking thesensor signals S1' to S4' in a known fashion. The speed v₁, v₂ of thefront wheels VR, VL could also be evaluated as a reference quantity inthe EBV mode. The vehicle reference speed is designated as v_(REF) inFIG. 2. V_(REF) is produced in a linking circuit 13. The vehiclereference speed v_(VREF) is differentiated in a circuit 14 to determinethe reference quantity b_(FZ) =b_(REF).

The deceleration of the rear wheels is produced similarly by way ofdifferentiating circuits 15 and 16. The working cycle for thedifferentiating circuits 15 and 16 is furnished by a clock pulsegenerator or timer 17 in the embodiment shown.

The deceleration signals b₃, b₄, representative of the rear-wheeldeceleration, are respectively sent to a differentiator and integratormeans 20, 21 by way of a filter 18, 19, provided as a low-pass filterand used as a deceleration element and for the suppression ofinterferences. The discrepancy of the deceleration of a rear wheel b₃,b₄ and b'₃, b'₄, respectively, from the vehicle deceleration b_(FZ) isdetermined in each case. The output signals x₁ and x₂ of thedifferentiators and integrators 20, 21 are produced according to therelation ##EQU1##

In the embodiment shown, x₁ refers to the right rear wheel HR, and x₂refers to the left rear wheel HL.

The output signals x(t) and x₁, x₂ of the differentiators andintegrators 20, 21, which constantly determine the difference betweenthe filtered rear-wheel deceleration b'₃, b'₄ and the vehicledeceleration b_(REF) and add it to, or subtract it from, the measuredvalue determined in the preceding cycle, are then compared withpredetermined limit values y₁, y₂, y₃ in comparators 22 to 27. y₁ >y₂>y₃ applies in terms of magnitude. The comparison for the output signalof the differentiator and integrator 20, i.e. for the right rear wheel,is performed in comparators 22 to 24 and for the left rear wheel incomparators 25 to 27.

The evaluation of the output signals of the comparators 22 to 27 invalve actuation circuits 28, 29 commences as soon as a circuit 30indicates an EBV function. The precondition for an EBV function is theexistence of a vehicle deceleration signal (-b_(FZ)) and a brake slipsignal.

If, upon the detection of the EBV function, the integrator content 20,21 or its output signal x(t), i.e. x₁ or x₂, becomes greater in terms ofmagnitude than the limit value y₂, the result is that the brakingpressure in the rear-wheel brake is maintained constant in the event offurther increase of the braking pressure in the front-wheel brake. Ifthe magnitude of the output signal x₁ exceeds the limit value y₁, and y₁>y₂ in terms of magnitude, this fact results in a reduction of thebraking pressure in the respective rear-wheel brake. If, finally, theoutput signal x₁ of the wheel stabilized by the braking pressurereduction falls below the limit value y₃, a further increase of thebraking pressure in the respective rear-wheel brake on command ofpressure increase pulses is permitted.

FIGS. 3A, 3B and 3C show waveform diagrams which illustrate the EBVfunction. Brake application commences at time t₀ in the situation towhich FIGS. 3A to 3C relate. FIG. 3A illustrates the vehicle referencespeed v_(REF) which represents the vehicle speed in approximation, andthe speed variation v_(HR) of a rear wheel in this EBV situation. As isshown in FIG. 3B, the negative signal x₁ at the time t₁ exceeds thepredetermined limit value y₂. At time t₁, the vehicle slows down andbrake slip prevails so that circuit 30 identifies an EBV function.According to FIG. 3C, the result of exceeding the limit value y₂ is thatthe rear-axle braking pressure _(PHR) is maintained constant. At timet₂, even the limit value y₁ is exceeded, and braking pressure reductionis thereby induced as shown by FIG. 3C. At time t₃, the limit value y₃is reached so that a new pulsed pressure increase is permitted tocommence in the brake of the right-rear wheel _(PHR). Pressuremaintenance in the rear wheel commences again at time t₄, succeeded by apressure reduction at time t₅ and, finally, pressure re-increase at timet₆. Brake application is terminated at time t₇.

FIG. 3C, showing the braking pressure variation on the front axle p_(VA)and the rear axle p_(HA) in the illustrated example and thecharacteristic curve of the pressure variation in the event of idealbraking pressure distribution to the front and the rear axles, showsthat the ideal distribution is approached sufficiently due to the EBVfunction.

The pressure variation p_(HR) in the right rear wheel is identical withthe pressure variation of the second rear wheel in the embodiment ofFIG. 3C. This condition was reached by a select-low circuit 31 in FIG. 2which ensures that the rear wheel having the lower braking pressurealways leads the pressure variation p_(HA) on the rear axle.Principally, however, an individual braking pressure control in the EBVmode is also possible in the wheel brakes of the rear-axle wheels by wayof the circuit according to the present invention.

The output signals of the select-low circuit 31 of FIG. 2 are finallysent to the symbolically represented rear-wheel valves 32, 33 whichdetermine the actual braking pressure variation in the rear-wheelbrakes. One output of the circuit 31 is sufficient to actuate theinlet/outlet valves 32, 33 of both rear-wheel brakes in a pureselect-low control. Separate outlets of the valve actuation circuit 31are of course required in certain situations to control the rear-wheelbraking pressure on each individual wheel, as has been shown.

The control of brake force distribution by way of the circuitarrangement according to the present invention on the basis of thedifference between the vehicle deceleration and the rear-wheeldeceleration is appropriately combined with the per se knownslip-responsive control. Basically, priority is conceded to that type ofcontrol which permits the greater control accuracy and safety inresponse to the respective situation, i.e., as a function of a high orlow coefficient of friction, homogeneous or different coefficientright/left, cornering or straight travel, etc.

We claim:
 1. A circuit arrangement for a vehicle having front wheels,rear wheels, brakes associated with the front wheels and the rearwheels, and a brake system with adjustment of the brake forcedistribution to the brakes associated with the rear wheels, said circuitarrangement comprising:electrically controllable hydraulic valvesinserted in pressure fluid conduits leading to and from the front-wheelbrakes and the rear-wheel brakes; means for developing indications ofthe speed of the front wheels and the rear wheels of the vehicle in theform of front-wheel and rear-wheel speed signals; and valve controlmeans responsive to the front-wheel speed indications and the rear-wheelspeed indications:(A) for:(a) determining deceleration values of therear wheels in the form of rear-wheel deceleration signals, (b)determining at least one of:(1) a deceleration value of the vehicle inthe form of a vehicle deceleration signal, and (2) a reference quantityrepresentative of the approximate deceleration value of the vehicle inthe form of an approximate deceleration signal, (c) comparing thedeceleration values of the rear wheels with at least one of:(1) thedeceleration value of the vehicle, and (2) the reference quantityrepresentative of the approximate deceleration value of the vehicle, todevelop comparison output signals, (d) evaluating a discrepancy of therear-wheel deceleration values from at least one of:(1) the vehicledeceleration value, and (2) the reference quantity representative of theapproximate deceleration value of the vehicle, in the form ofdiscrepancy signals, (e) generating, in response to a discrepancyevaluation, controls to actuate said electrically controllable hydraulicvalves for control of brake force distribution, and (B) including:(a)first and second differentiating circuits to which a first of therear-wheel speed signals and a second of the rear wheel speed signals,respectively, are supplied, (b) first and second low-pass filters towhich output signals of said first and second differentiating circuits,respectively, are supplied, and (c) differentiator and integrator meansfor:(1) comparing output signals of said first and second low-passfilters, respectively, with at least one of: (i) the vehicledeceleration signal, and (ii) the approximate deceleration signal, and(2) developing the comparison output signals from which the controlsignals are developed to reduce, keep constant or increase furthercontrol of the braking pressure in the rear-wheel brakes as a functionof the comparison output signals.
 2. A circuit arrangement according toclaim 1 wherein the comparison output signals of said differentiator andintegrator means are developed according to the relation:

    x(t)=∫(b'.sub.R -b)dt.

where:b'_(R) is the filtered deceleration signal of the right or leftrear wheel, and b_(FZ) is the vehicle deceleration or a referencequantity in approximation of the vehicle deceleration.
 3. A circuitarrangement according to claim 2 further including means for comparingthe comparison output signals of the differentiator and integrator meanswith first, second and third predetermined limit values, with said firstpredetermined limit value greater than said second predetermined limitvalue and said second predetermined limit value greater than said thirdpredetermined limit value, and when:(a) the comparison output signalsexceed the first predetermined limit value, the control signals causethe braking pressure in the rear-wheels brakes to be reduced, (b) thecomparison output signals exceed the second predetermined limit value,the control signals cause the braking pressure in the rear-wheels brakesto be maintained constant, and (c) the comparison output signals exceedthe third predetermined limit value, the control signals cause thebraking pressure in the rear-wheels brakes to be increased further.
 4. Acircuit arrangement according to claim 3 wherein deceleration of thefront wheels is the reference quantity representative of the approximatedeceleration value of the vehicle.
 5. A circuit arrangement according toclaim 4 wherein the braking pressure in the rear-wheel brakes iscontrolled individually for each wheel.
 6. A circuit arrangementaccording to claim 5 wherein the braking pressure in the rear-wheelbrakes is controlled jointly according to the select-low principle andthe braking pressure in the two rear-wheel brakes is dictated by thewheel having the lower braking pressure.
 7. A circuit arrangementaccording to claim 6 further including means for determining thedifference between the wheel slip of the rear wheels and the wheel slipof the front wheels as an additional control quantity for the brakeforce distribution.